The phenomenon of nuclear magnetic resonance (Nuclear Magnetic Resonance, NMR) is applied in fields, such as physics, chemistry, material science, life science and medical science, shortly after being found in 1946. In 1950s, the nuclear magnetic resonance came into use in the oil and gas industry, and originally applied to the field of oil reservoir rock physics. A nuclear magnetic resonance logging tool can utilize the nuclear magnetic resonance principle to detect stratum information around the borehole, and have unique capabilities of qualitative recognition and quantitative evaluation for the reservoir fluid.
A probe is one of the important parts in the nuclear magnetic resonance logging tool, and the structure of the probe determines key performances, such as a measuring mode of the tool, a nuclear magnetic resonance region and nuclear magnetic resonance signal intensity. The probe of nuclear magnetic resonance logging tool mainly includes a magnet and an antenna, the magnet can form a static magnetic field for polarizing spinning hydrogen protons, and the antenna can emit a radio frequency field for turning the spinning hydrogen protons, after the radio frequency field is removed, the spinning hydrogen protons start to precess along the static magnetic field, thus generate nuclear magnetic resonance inductive signals, and the stratum conditions can be analyzed by detecting the nuclear magnetic resonance inductive signals.
The existing probe of nuclear magnetic resonance logging tool usually adopts a column-shaped magnet, rounded sides of the magnet are an N pole and an S pole, respectively, the magnetic field distribution is formed by closed magnetic lines of force pointing from the N pole to the S pole, the antenna surrounds the magnet, and can excite polarized stratum regions all around (360 degrees) the borehole, so that there is no detecting blind zone around the borehole, a multi-frequency multi-slice measurement can be performed, but the measuring signal is only an average signal of signals in the 360-degree stratum. Accordingly, the probe of nuclear magnetic resonance logging tool in the prior art only can perform signal detection at radial depth dimension and axial depth dimension, but have no capability to detect signals in the circumferential multi-azimuth sensitive area.